1. Field of the Invention
The present invention relates to an AM-FM receiver having means for preventing shock noises that are generated during switching between AM and FM modes.
2. Prior Art
In the conventional AM-FM receiver shown in FIG. 1, the high frequency stages (the FM and AM front end circuit 3 and 4) and subsequent stages have a B+ bias voltage applied thereto at all times. Switching between AM and FM modes is accomplished by throwing a switch S.sub.1 for applying the B+ voltage in synchronism with a switch S.sub.2 which shifts between the AM and FM audio outputs. When the throwing operations of switches S.sub.1 and S.sub.2 are out of synchronism, shock noises are generated.
More specifically, if the switch S.sub.2 is thrown after the switch S.sub.1 when the receiver is switched from FM to AM, a positive or negative DC voltage is produced in a discriminator 7 if the receiver is out of tune with an FM station. At this time, since the B+ voltage applied to the FM front end circuit 3 is cancelled and the DC voltage is also eliminated, a shock noise is generated as shown in FIG. 2A. On the other hand, if the switch S.sub.2 is thrown before the switch S.sub.1 when the receiver is to switch from AM to FM, the B+ voltage is suddenly applied to the FM front end circuit 3, with the receiver out of tune, and a positive or negative DC voltage is generated in the discriminator 7. A shock noise is then produced as shown in FIG. 2B.